1. Technical Field
The present disclosure relates generally to microwave applicator probes used in tissue ablation procedures. More particularly, the present disclosure is directed to a microwave probe which can be tuned during ablation procedures to obtain a desired impedance match.
2. Background of Related Art
Treatment of certain diseases requires destruction of malignant tissue growths (e.g., tumors). It is known that tumor cells denature at elevated temperatures that are slightly lower than temperatures injurious to surrounding healthy cells. Therefore, known treatment methods, such as hyperthermia therapy, heat tumor cells to temperatures above 41° C., while maintaining adjacent healthy cells at lower temperatures to avoid irreversible cell damage. Such methods involve applying electromagnetic radiation to heat tissue and include ablation and coagulation of tissue. In particular, microwave energy is used to coagulate and/or ablate tissue to denature or kill the cancerous cells.
Microwave energy is applied via microwave ablation antenna probes which penetrate tissue to reach tumors. There are several types of microwave probes, such as monopole, dipole, and helical. In monopole and dipole probes, microwave energy radiates perpendicularly from the axis of the conductor. Monopole probe (e.g., antenna) includes a single, elongated microwave conductor surrounded by a dielectric sleeve, having a conductor exposed at the end of the probe. Dipole probes have a coaxial construction including an inner conductor and an outer conductor separated by a dielectric portion. More specifically, dipole microwave antennas have a long, thin inner conductor which extends along a longitudinal axis of the probe and is surrounded by an outer conductor. In certain variations, a portion or portions of the outer conductor may be selectively removed to provide for more effective outward radiation of energy. This type of microwave probe construction is typically referred to as a “leaky waveguide” or “leaky coaxial” antenna.
In helical probes, microwave energy is directed in a forward direction. This is due to microwave energy radiating perpendicularly from the antenna, which when in helical configuration directs the energy waves in a forward direction. In helical probes the inner conductor is formed in a uniform spiral pattern (e.g., a helix) to provide the required configuration for effective radiation.
Conventional microwave probes have a narrow operational bandwidth, a wavelength range at which optimal operational efficiency is achieved, and hence, are incapable of maintaining a predetermined impedance match between the microwave delivery system (e.g., generator, cable, etc.) and the tissue surrounding the microwave probe. More specifically, as microwave energy is applied to tissue, the dielectric constant of the tissue immediately surrounding the microwave probe decreases as the tissue is cooked. The drop causes the wavelength of the microwave energy being applied to tissue to increase beyond the bandwidth of the probe. As a result, there is a mismatch between the bandwidth of conventional microwave probe and the microwave energy being applied. Thus, narrow band microwave probes may detune as a result of steam generation and phase transformation of the tissue hindering effective energy delivery and dispersion.